Among conventional image forming devices having replaceable developer cartridges in which developer is stored, a device configured to determine the time to replace the attached developer cartridge is known. For example, an image forming device disclosed in Patent Document 1 (Japan Patent Application Laid-Open Publication No. H02-39178) starts to count the number of dots printed on sheet S right after replacement of the developer cartridge; calculates the total amount of developer consumed based on the number of dots and based on an amount of developer consumed that is required to form one dot; and determines that it is time to replace the developer cartridge if the calculated total amount of developer consumed exceeds a threshold value.